Integrated circuits often require an internal voltage that is different from the external voltage which is provided to the integrated circuit at the power supply input. This internal voltage is oftentimes not known ahead of time. In fact, this internal voltage is often determined during the actual testing of the integrated circuit itself.
To simplify the task of selecting an internal, or reference voltage, voltage reference circuits are typically designed into the power supply part of an integrated circuit. These voltage reference circuits are essentially voltage divider circuits, wherein branches of resistors of varying resistances are available to provide a scaled down voltage.
During the testing stage of integrated circuit production, combinations of branches of resistors are tested to achieve an optimum voltage level. When a desired combination is found, it is selected by either burning one or several fuses, or by adjusting a metal mask to permanently select the combination. These methods suffer from being inflexible, since programming with fuses or metal masks is a one-time only event and cannot be modified should a different optimum voltage level later be desired. Another disadvantage is that oftentimes a fuse is blown before the optimum voltage is reached.
One way of solving the problem of inflexibility associated with programming an optimum voltage level with fuses or metal masks is to use transistor programmability. An example of this prior art method is shown in FIG. 2. In FIG. 2, the top four p-channel transistors 20-23 each have their respective gates tied to ground and are thus always in the on state. In this configuration, each transistor 20-23 acts as a resistor whose resistance value is determined by the area of the respective transistor channel. One or a combination of the four transistor/resistors 20-23 are selected by selecting one or a combination of n-channel switching transistors 30-33 and n-channel enable switch transistors 34-37, which are connected in series with the transistor/resistors 20-23. A disadvantage of using this prior art type of transistor programming is that it occupies a substantial amount of area on the integrated circuit due to the fact that it uses both n-channel and p-channel transistors. See also, Cordoba, Hardee and Butler, U.S. Pat. No. 5,315,230 entitled "Temperature Compensated Voltage Reference For Low and Wide Voltage Ranges" issued May 24, 1994.